In vivo cell-autonomous transcriptional abnormalities revealed in mice expressing mutant huntingtin in striatal but not cortical neurons

Huntington's disease cellular phenotypes are rescued non-cell autonomously by healthy cells in mosaic telencephalic organoids

Huntington's disease (HD) causes selective degeneration of striatal and cortical neurons, resulting in cell mosaicism of coexisting still functional and dysfunctional cells. The impact of non-cell autonomous mechanisms between these cellular states is poorly understood. Here we generated telencephalic organoids with healthy or HD cells, grown separately or as mosaics of the two genotypes. Single-cell RNA sequencing revealed neurodevelopmental abnormalities in the ventral fate acquisition of HD organoids, confirmed by cytoarchitectural and transcriptional defects leading to fewer GABAergic neurons, while dorsal populations showed milder phenotypes mainly in maturation trajectory. Healthy cells in mosaic organoids restored HD cell identity, trajectories, synaptic density, and communication pathways upon cell-cell contact, while showing no significant alterations when grown with HD cells. These findings highlight cell-type-specific alterations in HD and beneficial non-cell autonomous effects of healthy cells, emphasizing the therapeutic potential of modulating cell-cell communication in disease progression and treatment.

The Neurodevelopmental Hypothesis of Huntington's Disease

The current dogma of HD pathoetiology posits it is a degenerative disease affecting primarily the striatum, caused by a gain of function (toxicity) of the mutant mHTT that kills neurons. However, a growing body of evidence supports an alternative theory in which loss of function may also influence the pathology.This theory is predicated on the notion that HTT is known to be a vital gene for brain development. mHTT is expressed throughout life and could conceivably have deleterious effects on brain development. The end event in the disease is, of course, neurodegeneration; however the process by which that occurs may be rooted in the pathophysiology of aberrant development.To date, there have been multiple studies evaluating molecular and cellular mechanisms of abnormal development in HD, as well as studies investigating abnormal brain development in HD animal models. However, direct study of how mHTT could affect neurodevelopment in humans has not been approached until recent years. The current review will focus on the most recent findings of a unique study of children at-risk for HD, the Kids-HD study. This study evaluates brain structure and function in children ages 6-18 years old who are at risk for HD (have a parent or grand-parent with HD).

Dysregulation of PGC-1α-Dependent Transcriptional Programs in Neurological and Developmental Disorders: Therapeutic Challenges and Opportunities

Substantial evidence indicates that mitochondrial impairment contributes to neuronal dysfunction and vulnerability in disease states, leading investigators to propose that the enhancement of mitochondrial function should be considered a strategy for neuroprotection. However, multiple attempts to improve mitochondrial function have failed to impact disease progression, suggesting that the biology underlying the normal regulation of mitochondrial pathways in neurons, and its dysfunction in disease, is more complex than initially thought. Here, we present the proteins and associated pathways involved in the transcriptional regulation of nuclear-encoded genes for mitochondrial function, with a focus on the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1α). We highlight PGC-1α’s roles in neuronal and non-neuronal cell types and discuss evidence for the dysregulation of PGC-1α-dependent pathways in Huntington’s Disease, Parkinson’s Disease, and developmental disorders, emphasizing the relationship between disease-specific cellular vulnerability and cell-type-specific patterns of PGC-1α expression. Finally, we discuss the challenges inherent to therapeutic targeting of PGC-1α-related transcriptional programs, considering the roles for neuron-enriched transcriptional coactivators in co-regulating mitochondrial and synaptic genes. This information will provide novel insights into the unique aspects of transcriptional regulation of mitochondrial function in neurons and the opportunities for therapeutic targeting of transcriptional pathways for neuroprotection.

Dysregulation of PGC-1α-Dependent Transcriptional Programs in Neurological and Developmental Disorders: Therapeutic Challenges and Opportunities

Substantial evidence indicates that mitochondrial impairment contributes to neuronal dysfunction and vulnerability in disease states, leading investigators to propose that the enhancement of mitochondrial function should be considered a strategy for neuroprotection. However, multiple attempts to improve mitochondrial function have failed to impact disease progression, suggesting that the biology underlying the normal regulation of mitochondrial pathways in neurons, and its dysfunction in disease, is more complex than initially thought. Here, we present the proteins and associated pathways involved in the transcriptional regulation of nuclear-encoded genes for mitochondrial function, with a focus on the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1α). We highlight PGC-1α's roles in neuronal and non-neuronal cell types and discuss evidence for the dysregulation of PGC-1α-dependent pathways in Huntington's Disease, Parkinson's Disease, and developmental disorders, emphasizing the relationship between disease-specific cellular vulnerability and cell-type-specific patterns of PGC-1α expression. Finally, we discuss the challenges inherent to therapeutic targeting of PGC-1α-related transcriptional programs, considering the roles for neuron-enriched transcriptional coactivators in co-regulating mitochondrial and synaptic genes. This information will provide novel insights into the unique aspects of transcriptional regulation of mitochondrial function in neurons and the opportunities for therapeutic targeting of transcriptional pathways for neuroprotection.

Selective Neuronal Death in Neurodegenerative Diseases: The Ongoing Mystery

A major unresolved problem in neurodegenerative disease is why and how a specific set of neurons in the brain are highly vulnerable to neuronal death. Multiple pathways and mechanisms have been proposed to play a role in Alzheimer disease (AD), Parkinson disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington disease (HD), yet how they contribute to neuronal vulnerability remains far from clear. In this review, various mechanisms ascribed in AD, PD, ALS, and HD will be briefly summarized. Particular focus will be placed on Rhes-mediated intercellular transport of the HD protein and its role in mitophagy, in which I will discuss some intriguing observations that I apply to model striatal vulnerability in HD. I may have unintentionally missed referring some studies in this review, and I extend my apologies to the authors in those circumstances.

Cell-Autonomous and Non-cell-Autonomous Pathogenic Mechanisms in Huntington's Disease: Insights from In Vitro and In Vivo Models

Huntington's disease (HD) is an autosomal dominant disorder caused by an expansion in the trinucleotide CAG repeat in exon-1 in the huntingtin gene, located on chromosome 4. When the number of trinucleotide CAG exceeds 40 repeats, disease invariably is manifested, characterized by motor, cognitive, and psychiatric symptoms. The huntingtin (Htt) protein and its mutant form (mutant huntingtin, mHtt) are ubiquitously expressed but although multiple brain regions are affected, the most vulnerable brain region is the striatum. Striatal medium-sized spiny neurons (MSNs) preferentially degenerate, followed by the cortical pyramidal neurons located in layers V and VI. Proposed HD pathogenic mechanisms include, but are not restricted to, excitotoxicity, neurotrophic support deficits, collapse of the protein degradation mechanisms, mitochondrial dysfunction, transcriptional alterations, and disorders of myelin. Studies performed in cell type-specific and regionally selective HD mouse models implicate both MSN cell-autonomous properties and cell-cell interactions, particularly corticostriatal but also with non-neuronal cell types. Here, we review the intrinsic properties of MSNs that contribute to their selective vulnerability and in addition, we discuss how astrocytes, microglia, and oligodendrocytes, together with aberrant corticostriatal connectivity, contribute to HD pathophysiology. In addition, mHtt causes cell-autonomous dysfunction in cell types other than MSNs. These findings have implications in terms of therapeutic strategies aimed at preventing neuronal dysfunction and degeneration.

Environment-dependent striatal gene expression in the BACHD rat model for Huntington disease

Huntington disease (HD) is an autosomal dominant neurodegenerative disorder caused by a mutation in the huntingtin (HTT) gene which results in progressive neurodegeneration in the striatum, cortex, and eventually most brain areas. Despite being a monogenic disorder, environmental factors influence HD characteristics. Both human and mouse studies suggest that mutant HTT (mHTT) leads to gene expression changes that harbor potential to be modulated by the environment. Yet, the underlying mechanisms integrating environmental cues into the gene regulatory program have remained largely unclear. To better understand gene-environment interactions in the context of mHTT, we employed RNA-seq to examine effects of maternal separation (MS) and environmental enrichment (EE) on striatal gene expression during development of BACHD rats. We integrated our results with striatal consensus modules defined on HTT-CAG length and age-dependent co-expression gene networks to relate the environmental factors with disease progression. While mHTT was the main determinant of expression changes, both MS and EE were capable of modulating these disturbances, resulting in distinctive and in several cases opposing effects of MS and EE on consensus modules. This bivalent response to maternal separation and environmental enrichment may aid in explaining their distinct effects observed on disease phenotypes in animal models of HD and related neurodegenerative disorders.

Biological function of Lemur tyrosine kinase 2 (LMTK2): implications in neurodegeneration

Neurodegenerative disorders are frequent, incurable diseases characterised by abnormal protein accumulation and progressive neuronal loss. Despite their growing prevalence, the underlying pathomechanism remains unclear. Lemur tyrosine kinase 2 (LMTK2) is a member of a transmembrane serine/threonine-protein kinase family. Although it was described more than a decade ago, our knowledge on LMTK2’s biological functions is still insufficient. Recent evidence has suggested that LMTK2 is implicated in neurodegeneration. After reviewing the literature, we identified three LMTK2-mediated mechanisms which may contribute to neurodegenerative processes: disrupted axonal transport, tau hyperphosphorylation and enhanced apoptosis. Moreover, LMTK2 gene expression is decreased in an Alzheimer’s disease mouse model. According to these features, LMTK2 might be a promising therapeutic target in near future. However, further investigations are required to clarify the exact biological functions of this unique protein.

Corticostriatal network dysfunction in Huntington's disease: Deficits in neural processing, glutamate transport, and ascorbate release

AIMS

This review summarizes evidence for dysfunctional connectivity between cortical and striatal neurons in Huntington's disease (HD), a fatal neurodegenerative condition caused by a single gene mutation. The focus is on data derived from recording of electrophysiological signals in behaving transgenic mouse models.

DISCUSSIONS

Firing patterns of individual neurons and the frequency oscillations of local field potentials indicate a disruption in corticostriatal processing driven, in large part, by interactions between cells that contain the mutant gene rather than the mutant gene alone. Dysregulation of glutamate, an excitatory amino acid released by cortical afferents, plays a key role in the breakdown of corticostriatal communication, a process modulated by ascorbate, an antioxidant vitamin found in high concentration in striatum. Up-regulation of glutamate transport by drug administration or viral-vector delivery improves ascorbate homeostasis and neurobehavioral processing in HD mice. Further analysis of electrophysiological data, including the use of sophisticated computational strategies, is required to discern how behavioral demands modulate the flow of corticostriatal information and its disruption by HD.

CONCLUSIONS

Long before massive cell loss occurs, HD impairs the mechanisms by which cortical and striatal neurons communicate. A key problem identified in transgenic animal models is dysregulation of the dynamic changes in extracellular glutamate and ascorbate. Improved understanding of how these neurochemical systems impact corticostriatal communication is necessary before an effective therapeutic strategy can emerge.

Molecular insights into cortico-striatal miscommunications in Huntington's disease

Huntington's disease (HD), a dominantly inherited neurodegenerative disease, is defined by its genetic cause, a CAG-repeat expansion in the HTT gene, its motor and psychiatric symptomology and primary loss of striatal medium spiny neurons (MSNs). However, the molecular mechanisms from genetic lesion to disease phenotype remain largely unclear. Mouse models of HD have been created that exhibit phenotypes partially recapitulating those in the patient, and specifically, cortico-striatal disconnectivity appears to be a shared pathogenic event shared by HD mouse models and patients. Molecular studies have begun to unveil converging molecular and cellular pathogenic mechanisms that may account for cortico-striatal miscommunication in various HD mouse models. Systems biological approaches help to illuminate synaptic molecular networks as a nexus for HD cortio-striatal pathogenesis, and may offer new candidate targets to modify the disease.

2,4 DNP improves motor function, preserves medium spiny neuronal identity, and reduces oxidative stress in a mouse model of Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the first exon of the gene huntingtin. There is no treatment to prevent or delay the disease course of HD currently. Oxidative stress and mitochondrial dysfunction have emerged as key determinants of the disease progression in HD. Therefore, counteracting mutant huntingtin (mHtt)-induced oxidative stress and mitochondrial dysfunction appears as a new approach to treat this devastating disease. Interestingly, mild mitochondrial uncoupling improves neuronal resistance to stress and facilitates neuronal survival. Mild mitochondrial uncoupling can be induced by the proper dose of 2,4-dinitrophenol (DNP), a proton ionophore that was previously used for weight loss. In this study, we evaluated the effects of chronic administration of DNP at three doses (0.5, 1, 5mg/kg/day) on mHtt-induced behavioral deficits and cellular abnormalities in the N171-82Q HD mouse model. DNP at a low dose (1mg/kg/day) significantly improved motor function and preserved medium spiny neuronal marker DARPP32 and postsynaptic protein PSD95 in the striatum of HD mice. Further mechanistic study suggests that DNP at this dose reduced oxidative stress in HD mice, which was indicated by reduced levels of F2-isoprostanes in the brain of HD mice treated with DNP. Our data indicated that DNP provided behavioral benefit and neuroprotective effect at a weight neutral dose in HD mice, suggesting that the potential value of repositioning DNP to HD treatment is warranted in well-controlled clinical trials in HD.

Synaptopathic mechanisms of neurodegeneration and dementia: Insights from Huntington's disease

Dementia encapsulates a set of symptoms that include loss of mental abilities such as memory, problem solving or language, and reduces a person's ability to perform daily activities. Alzheimer's disease is the most common form of dementia, however dementia can also occur in other neurological disorders such as Huntington's disease (HD). Many studies have demonstrated that loss of neuronal cell function manifests pre-symptomatically and thus is a relevant therapeutic target to alleviate symptoms. Synaptopathy, the physiological dysfunction of synapses, is now being approached as the target for many neurological and psychiatric disorders, including HD. HD is an autosomal dominant and progressive degenerative disorder, with clinical manifestations that encompass movement, cognition, mood and behaviour. HD is one of the most common tandem repeat disorders and is caused by a trinucleotide (CAG) repeat expansion, encoding an extended polyglutamine tract in the huntingtin protein. Animal models as well as human studies have provided detailed, although not exhaustive, evidence of synaptic dysfunction in HD. In this review, we discuss the neuropathology of HD and how the changes in synaptic signalling in the diseased brain lead to its symptoms, which include dementia. Here, we review and discuss the mechanisms by which the 'molecular orchestras' and their 'synaptic symphonies' are disrupted in neurodegeneration and dementia, focusing on HD as a model disease. We also explore the therapeutic strategies currently in pre-clinical and clinical testing that are targeted towards improving synaptic function in HD.

TRiC subunits enhance BDNF axonal transport and rescue striatal atrophy in Huntington's disease

Corticostriatal atrophy is a cardinal manifestation of Huntington's disease (HD). However, the mechanism(s) by which mutant huntingtin (mHTT) protein contributes to the degeneration of the corticostriatal circuit is not well understood. We recreated the corticostriatal circuit in microfluidic chambers, pairing cortical and striatal neurons from the BACHD model of HD and its WT control. There were reduced synaptic connectivity and atrophy of striatal neurons in cultures in which BACHD cortical and striatal neurons were paired. However, these changes were prevented if WT cortical neurons were paired with BACHD striatal neurons; synthesis and release of brain-derived neurotrophic factor (BDNF) from WT cortical axons were responsible. Consistent with these findings, there was a marked reduction in anterograde transport of BDNF in BACHD cortical neurons. Subunits of the cytosolic chaperonin T-complex 1 (TCP-1) ring complex (TRiC or CCT for chaperonin containing TCP-1) have been shown to reduce mHTT levels. Both CCT3 and the apical domain of CCT1 (ApiCCT1) decreased the level of mHTT in BACHD cortical neurons. In cortical axons, they normalized anterograde BDNF transport, restored retrograde BDNF transport, and normalized lysosomal transport. Importantly, treating BACHD cortical neurons with ApiCCT1 prevented BACHD striatal neuronal atrophy by enhancing release of BDNF that subsequently acts through tyrosine receptor kinase B (TrkB) receptor on striatal neurons. Our findings are evidence that TRiC reagent-mediated reductions in mHTT enhanced BDNF delivery to restore the trophic status of BACHD striatal neurons.

Transcriptome sequencing reveals aberrant alternative splicing in Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a CAG expansion in the gene-encoding Huntingtin (HTT). Transcriptome dysregulation is a major feature of HD pathogenesis, as revealed by a large body of work on gene expression profiling of tissues from human HD patients and mouse models. These studies were primarily focused on transcriptional changes affecting steady-state overall gene expression levels using microarray based approaches. A major missing component, however, has been the study of transcriptome changes at the post-transcriptional level, such as alternative splicing. Alternative splicing is a critical mechanism for expanding regulatory and functional diversity from a limited number of genes, and is particularly complex in the mammalian brain. Here we carried out a deep RNA-seq analysis of the BA4 (Brodmann area 4) motor cortex from seven human HD brains and seven controls to systematically discover aberrant alternative splicing events and characterize potential associated splicing factors in HD. We identified 593 differential alternative splicing events between HD and control brains. Using two expanded panels with a total of 108 BA4 tissues from patients and controls, we identified four splicing factors exhibiting significantly altered expression levels in HD patient brains. Moreover, follow-up molecular analyses of one splicing factor PTBP1 revealed its impact on disease-associated splicing patterns in HD. Collectively, our data provide genomic evidence for widespread splicing dysregulation in HD brains, and suggest the role of aberrant alternative splicing in the pathogenesis of HD.

Cortical efferents lacking mutant huntingtin improve striatal neuronal activity and behavior in a conditional mouse model of Huntington's disease

Abnormal electrophysiological activity in the striatum, which receives dense innervation from the cerebral cortex, is believed to set the stage for the behavioral phenotype observed in Huntington's disease (HD), a neurodegenerative condition caused by mutation of the huntingtin (mhtt) protein. However, cortical involvement is far from clear. To determine whether abnormal striatal processing can be explained by mhtt alone (cell-autonomous model) or by mhtt in the corticostriatal projection cell-cell interaction model, we used BACHD/Emx1-Cre (BE) mice, a conditional HD model in which full-length mhtt is genetically reduced in cortical output neurons, including those that project to the striatum. Animals were assessed beginning at 20 weeks of age for at least the next 40 weeks, a range over which presymptomatic BACHD mice become symptomatic. Both open-field and nest-building behavior deteriorated progressively in BACHD mice relative to both BE and wild-type (WT) mice. Neuronal activity patterns in the dorsal striatum, which receives input from the primary motor cortex (M1), followed a similar age progression because BACHD activity changed more rapidly than either BE or WT mice. However, in the M1, BE neuronal activity differed significantly from both WT and BACHD. Although abnormal cortical activity in BE mice likely reflects input from mhtt-expressing afferents, including cortical interneurons, improvements in BE striatal activity and behavior suggest a critical role for mhtt in cortical output neurons in shaping the onset and progression of striatal dysfunction.

Components of the endocannabinoid and dopamine systems are dysregulated in Huntington's disease: analysis of publicly available microarray datasets

The endocannabinoid system (ECS) and the dopaminergic system (DAS) are two major regulators of basal ganglia function. During Huntington's disease (HD) pathogenesis, the expression of genes in both the ECS and DAS is dysregulated. The purpose of this study was to determine the changes that were consistently observed in the ECS and DAS during HD progression in the central nervous system (CNS) and in the periphery in different models of HD and human HD tissue. To do this, we conducted a meta-analysis of differential gene expression in the ECS and DAS using publicly available microarray data. The consolidated data were summarized as observed changes in gene expression (OCGE) using a weighted sum for each gene. In addition, consolidated data were compared to previously published studies that were not available in the gene expression omnibus (GEO) database. The resulting data confirm gene expression changes observed using different approaches and provide novel insights into the consistency between changes observed in human tissue and various models, as well as disease stage- and tissue-specific transcriptional dysregulation in HD. The major implication of the systems-wide data presented here is that therapeutic strategies targeting the ECS or DAS must consider the dynamic changes in gene expression over time and in different body areas, which occur during HD progression and the interconnectedness of the two systems.

Hypothalamic alterations in Huntington's disease patients: comparison with genetic rodent models

Unintended weight loss, sleep and circadian disturbances and autonomic dysfunction are prevalent features of Huntington's disease (HD), an autosomal dominantly inherited neurodegenerative disorder caused by an expanded CAG repeat sequence in the HTT gene. These features form a substantial contribution to disease burden in HD patients and appear to be accompanied by a number of neuroendocrine and metabolic changes, pointing towards hypothalamic pathology as a likely underlying mechanism. Neuronal inclusion bodies of mutant huntingtin, which are hallmarks of the disease, occur throughout the hypothalamus, and indicate local mutant huntingtin expression that could interfere with hypothalamic neuropeptide production. Also, several genetic rodent models of HD show features that could be related to hypothalamic pathology, such as weight loss and circadian rhythm disturbances. In these rodents, several hypothalamic neuropeptide populations are affected. In the present review, we summarise the changes in genetic rodent models of HD for individual hypothalamic nuclei, compare these observations to the hypothalamic changes that occur in HD patients, and make an inventory of the work that still needs to be done. Surprisingly, there is only limited overlap in the hypothalamic changes reported in HD patients and genetic rodent models. At present, the only similarity between the hypothalamic alterations in HD patients and genetic rodent models is a decrease in the number of orexin-expressing neurones in the lateral hypothalamus. Possible reasons for these discrepancies, as well as potential consequences for the development of novel therapeutic strategies, are discussed.

The Wnt receptor Ryk reduces neuronal and cell survival capacity by repressing FOXO activity during the early phases of mutant huntingtin pathogenicity

The Wnt receptor Ryk is an evolutionary-conserved protein important during neuronal differentiation through several mechanisms, including γ-secretase cleavage and nuclear translocation of its intracellular domain (Ryk-ICD). Although the Wnt pathway may be neuroprotective, the role of Ryk in neurodegenerative disease remains unknown. We found that Ryk is up-regulated in neurons expressing mutant huntingtin (HTT) in several models of Huntington's disease (HD). Further investigation in Caenorhabditis elegans and mouse striatal cell models of HD provided a model in which the early-stage increase of Ryk promotes neuronal dysfunction by repressing the neuroprotective activity of the longevity-promoting factor FOXO through a noncanonical mechanism that implicates the Ryk-ICD fragment and its binding to the FOXO co-factor β-catenin. The Ryk-ICD fragment suppressed neuroprotection by lin-18/Ryk loss-of-function in expanded-polyQ nematodes, repressed FOXO transcriptional activity, and abolished β-catenin protection of mutant htt striatal cells against cell death vulnerability. Additionally, Ryk-ICD was increased in the nucleus of mutant htt cells, and reducing γ-secretase PS1 levels compensated for the cytotoxicity of full-length Ryk in these cells. These findings reveal that the Ryk-ICD pathway may impair FOXO protective activity in mutant polyglutamine neurons, suggesting that neurons are unable to efficiently maintain function and resist disease from the earliest phases of the pathogenic process in HD.

Transcription, epigenetics and ameliorative strategies in Huntington's Disease: a genome-wide perspective

Transcriptional dysregulation in Huntington’s disease (HD) is an early event that shapes the brain transcriptome by both the depletion and ectopic activation of gene products that eventually affect survival and neuronal functions. Disruption in the activity of gene expression regulators, such as transcription factors, chromatin-remodeling proteins, and noncoding RNAs, accounts for the expression changes observed in multiple animal and cellular models of HD and in samples from patients. Here, I review the recent advances in the study of HD transcriptional dysregulation and its causes to finally discuss the possible implications in ameliorative strategies from a genome-wide perspective. To date, the use of genome-wide approaches, predominantly based on microarray platforms, has been successful in providing an extensive catalog of differentially regulated genes, including biomarkers aimed at monitoring the progress of the pathology. Although still incipient, the introduction of combined next-generation sequencing techniques is enhancing our comprehension of the mechanisms underlying altered transcriptional dysregulation in HD by providing the first genomic landscapes associated with epigenetics and the occupancy of transcription factors. In addition, the use of genome-wide approaches is becoming more and more necessary to evaluate the efficacy and safety of ameliorative strategies and to identify novel mechanisms of amelioration that may help in the improvement of current preclinical therapeutics. Finally, the major conclusions obtained from HD transcriptomics studies have the potential to be extrapolated to other neurodegenerative disorders.

A novel cognitive-neurophysiological state biomarker in premanifest Huntington's disease validated on longitudinal data

In several neurodegenerative diseases, like Huntington's disease (HD), treatments are still lacking. To determine whether a treatment is effective, sensitive disease progression biomarkers are especially needed for the premanifest phase, since this allows the evaluation of neuroprotective treatments preventing, or delaying disease manifestation. On the basis of a longitudinal study we present a biomarker that was derived by integrating behavioural and neurophysiological data reflecting cognitive processes of action control. The measure identified is sensitive enough to track disease progression over a period of only 6 month. Changes tracked were predictive for a number of clinically relevant parameters and the sensitivity of the measure was higher than that of currently used parameters to track prodromal disease progression. The study provides a biomarker, which could change practice of progression diagnostics in a major basal ganglia disease and which may help to evaluate potential neuroprotective treatments in future clinical trials.

HACE1 reduces oxidative stress and mutant Huntingtin toxicity by promoting the NRF2 response

Oxidative stress plays a key role in late onset diseases including cancer and neurodegenerative diseases such as Huntington disease. Therefore, uncovering regulators of the antioxidant stress responses is important for understanding the course of these diseases. Indeed, the nuclear factor erythroid 2-related factor 2 (NRF2), a master regulator of the cellular antioxidative stress response, is deregulated in both cancer and neurodegeneration. Similar to NRF2, the tumor suppressor Homologous to the E6-AP Carboxyl Terminus (HECT) domain and Ankyrin repeat containing E3 ubiquitin-protein ligase 1 (HACE1) plays a protective role against stress-induced tumorigenesis in mice, but its roles in the antioxidative stress response or its involvement in neurodegeneration have not been investigated. To this end we examined Hace1 WT and KO mice and found that Hace1 KO animals exhibited increased oxidative stress in brain and that the antioxidative stress response was impaired. Moreover, HACE1 was found to be essential for optimal NRF2 activation in cells challenged with oxidative stress, as HACE1 depletion resulted in reduced NRF2 activity, stability, and protein synthesis, leading to lower tolerance against oxidative stress triggers. Strikingly, we found a reduction of HACE1 levels in the striatum of Huntington disease patients, implicating HACE1 in the pathology of Huntington disease. Moreover, ectopic expression of HACE1 in striatal neuronal progenitor cells provided protection against mutant Huntingtin-induced redox imbalance and hypersensitivity to oxidative stress, by augmenting NRF2 functions. These findings reveal that the tumor suppressor HACE1 plays a role in the NRF2 antioxidative stress response pathway and in neurodegeneration.

Maintenance of basal levels of autophagy in Huntington's disease mouse models displaying metabolic dysfunction

Huntington's disease (HD) is a fatal neurodegenerative disorder caused by an expanded polyglutamine repeat in the huntingtin protein. Neuropathology in the basal ganglia and in the cerebral cortex has been linked to the motor and cognitive symptoms whereas recent work has suggested that the hypothalamus might be involved in the metabolic dysfunction. Several mouse models of HD that display metabolic dysfunction have hypothalamic pathology, and expression of mutant huntingtin in the hypothalamus has been causally linked to the development of metabolic dysfunction in mice. Although the pathogenic mechanisms by which mutant huntingtin exerts its toxic functions in the HD brain are not fully known, several studies have implicated a role for the lysososomal degradation pathway of autophagy. Interestingly, changes in autophagy in the hypothalamus have been associated with the development of metabolic dysfunction in wild-type mice. We hypothesized that expression of mutant huntingtin might lead to changes in the autophagy pathway in the hypothalamus in mice with metabolic dysfunction. We therefore investigated whether there were changes in basal levels of autophagy in a mouse model expressing a fragment of 853 amino acids of mutant huntingtin selectively in the hypothalamus using a recombinant adeno-associate viral vector approach as well as in the transgenic BACHD mice. We performed qRT-PCR and Western blot to investigate the mRNA and protein expression levels of selected autophagy markers. Our results show that basal levels of autophagy are maintained in the hypothalamus despite the presence of metabolic dysfunction in both mouse models. Furthermore, although there were no major changes in autophagy in the striatum and cortex of BACHD mice, we detected modest, but significant differences in levels of some markers in mice at 12 months of age. Taken together, our results indicate that overexpression of mutant huntingtin in mice do not significantly perturb basal levels of autophagy.

Mutant huntingtin gene-dose impacts on aggregate deposition, DARPP32 expression and neuroinflammation in HdhQ150 mice

Huntington's disease (HD) is an autosomal dominant, progressive and fatal neurological disorder caused by an expansion of CAG repeats in exon-1 of the huntingtin gene. The encoded poly-glutamine stretch renders mutant huntingtin prone to aggregation. HdhQ150 mice genocopy a pathogenic repeat (∼150 CAGs) in the endogenous mouse huntingtin gene and model predominantly pre-manifest HD. Treating early is likely important to prevent or delay HD, and HdhQ150 mice may be useful to assess therapeutic strategies targeting pre-manifest HD. This requires appropriate markers and here we demonstrate, that pre-symptomatic HdhQ150 mice show several dramatic mutant huntingtin gene-dose dependent pathological changes including: (i) an increase of neuronal intra-nuclear inclusions (NIIs) in brain, (ii) an increase of extra-nuclear aggregates in dentate gyrus, (iii) a decrease of DARPP32 protein and (iv) an increase in glial markers of neuroinflammation, which curiously did not correlate with local neuronal mutant huntingtin inclusion-burden. HdhQ150 mice developed NIIs also in all retinal neuron cell-types, demonstrating that retinal NIIs are not specific to human exon-1 R6 HD mouse models. Taken together, the striking and robust mutant huntingtin gene-dose related changes in aggregate-load, DARPP32 levels and glial activation markers should greatly facilitate future testing of therapeutic strategies in the HdhQ150 HD mouse model.

Genetic manipulations of mutant huntingtin in mice: new insights into Huntington's disease pathogenesis

This year (2013) marks the 20th anniversary of identification of the causal genetic mutation for Huntington's disease (HD), a landmark discovery that heralded study of the biological underpinnings of this most common dominantly inherited neurodegenerative disorder. Among the variety of model organisms used to study HD pathogenesis, the mouse model is by far the most commonly used mammalian genetic model. Much of our current knowledge regarding mutant huntingtin (mHtt)-induced disease pathogenesis in mammalian models has been obtained by studying transgenic mouse models expressing mHtt N-terminal fragments, full-length murine or human mHtt. In this review, we focus on recent progress in using novel HD mouse models with targeted mHtt expression in specific brain cell types. These models help to address the role of distinct neuronal and non-neuronal cell types in eliciting cell-autonomous or non-cell-autonomous disease processes in HD. We also describe several mHtt transgenic mouse models with targeted mutations in Htt cis-domains to address specific pathogenic hypotheses, ranging from mHtt proteolysis to post-translational modifications. These novel mouse genetic studies, through direct manipulations of the causal HD gene, utilize a reductionist approach to systematically unravel the cellular and molecular pathways that are targeted by mHtt in disease pathogenesis, and to potentially identify novel targets for therapy.

Pathways to decoding the clinical potential of stress response FOXO-interaction networks for Huntington's disease: of gene prioritization and context dependence

The FOXO family of transcription factors is central to the regulation of organismal longevity and cellular survival. Several studies have indicated that FOXO factors lie at the center of a complex network of upstream pathways, cofactors and downstream targets (FOXO-interaction networks), which may have developmental and post-developmental roles in the regulation of chronic-stress response in normal and diseased cells. Noticeably, FOXO factors are important for the regulation of proteotoxicity and neuron survival in several models of neurodegenerative disease, suggesting that FOXO-interaction networks may have therapeutic potential. However, the status of FOXO-interaction networks in neurodegenerative disease remains largely unknown. Systems modeling is anticipated to provide a comprehensive assessment of this question. In particular, interrogating the context-dependent variability of FOXO-interaction networks could predict the clinical potential of cellular-stress response genes and aging regulators for tackling brain and peripheral pathology in neurodegenerative disease. Using published transcriptomic data obtained from murine models of Huntington's disease (HD) and post-mortem brains, blood samples and induced-pluripotent-stem cells from HD carriers as a case example, this review briefly highlights how the biological status and clinical potential of FOXO-interaction networks for HD may be decoded by developing network and entropy based feature selection across heterogeneous datasets.

Integration of genome-wide approaches identifies lncRNAs of adult neural stem cells and their progeny in vivo

Long noncoding RNAs (lncRNAs) have been described in cell lines and various whole tissues, but lncRNA analysis of development in vivo is limited. Here, we comprehensively analyze lncRNA expression for the adult mouse subventricular zone neural stem cell lineage. We utilize complementary genome-wide techniques including RNA-seq, RNA CaptureSeq, and ChIP-seq to associate specific lncRNAs with neural cell types, developmental processes, and human disease states. By integrating data from chromatin state maps, custom microarrays, and FACS purification of the subventricular zone lineage, we stringently identify lncRNAs with potential roles in adult neurogenesis. shRNA-mediated knockdown of two such lncRNAs, Six3os and Dlx1as, indicate roles for lncRNAs in the glial-neuronal lineage specification of multipotent adult stem cells. Our data and workflow thus provide a uniquely coherent in vivo lncRNA analysis and form the foundation of a user-friendly online resource for the study of lncRNAs in development and disease.

Integration of genome-wide approaches identifies lncRNAs of adult neural stem cells and their progeny in vivo

Long noncoding RNAs (lncRNAs) have been described in cell lines and various whole tissues, but lncRNA analysis of development in vivo is limited. Here, we comprehensively analyze lncRNA expression for the adult mouse subventricular zone neural stem cell lineage. We utilize complementary genome-wide techniques including RNA-seq, RNA CaptureSeq, and ChIP-seq to associate specific lncRNAs with neural cell types, developmental processes, and human disease states. By integrating data from chromatin state maps, custom microarrays, and FACS purification of the subventricular zone lineage, we stringently identify lncRNAs with potential roles in adult neurogenesis. shRNA-mediated knockdown of two such lncRNAs, Six3os and Dlx1as, indicate roles for lncRNAs in the glial-neuronal lineage specification of multipotent adult stem cells. Our data and workflow thus provide a uniquely coherent in vivo lncRNA analysis and form the foundation of a user-friendly online resource for the study of lncRNAs in development and disease.

Role of cerebral cortex in the neuropathology of Huntington's disease

An expansion of glutamine repeats in the N-terminal domain of the huntingtin protein leads to Huntington's disease (HD), a neurodegenerative condition characterized by the presence of involuntary movements, dementia, and psychiatric disturbances. Evaluation of postmortem HD tissue indicates that the most prominent cell loss occurs in cerebral cortex and striatum, forebrain regions in which cortical pyramidal neurons (CPNs) and striatal medium spiny neurons (MSNs) are the most affected. Subsequent evidence obtained from HD patients and especially from transgenic mouse models of HD indicates that long before neuronal death, patterns of communication between CPNs and MSNs become dysfunctional. In fact, electrophysiological signaling in transgenic HD mice is altered even before the appearance of the HD behavioral phenotype, suggesting that dysfunctional cortical input to the striatum sets the stage for the emergence of HD neurological signs. Striatal MSNs, moreover, project back to cortex via multi-synaptic connections, allowing for even further disruptions in cortical processing. An effective therapeutic strategy for HD, therefore, may lie in understanding the synaptic mechanisms by which it dysregulates the corticostriatal system. Here, we review literature evaluating the molecular, morphological, and physiological alterations in the cerebral cortex, a key component of brain circuitry controlling motor behavior, as they occur in both patients and transgenic HD models.

Mouse models of polyglutamine diseases: review and data table. Part I

Polyglutamine (polyQ) disorders share many similarities, such as a common mutation type in unrelated human causative genes, neurological character, and certain aspects of pathogenesis, including morphological and physiological neuronal alterations. The similarities in pathogenesis have been confirmed by findings that some experimental in vivo therapy approaches are effective in multiple models of polyQ disorders. Additionally, mouse models of polyQ diseases are often highly similar between diseases with respect to behavior and the features of the disease. The common features shared by polyQ mouse models may facilitate the investigation of polyQ disorders and may help researchers explore the mechanisms of these diseases in a broader context. To provide this context and to promote the understanding of polyQ disorders, we have collected and analyzed research data about the characterization and treatment of mouse models of polyQ diseases and organized them into two complementary Excel data tables. The data table that is presented in this review (Part I) covers the behavioral, molecular, cellular, and anatomic characteristics of polyQ mice and contains the most current knowledge about polyQ mouse models. The structure of this data table is designed in such a way that it can be filtered to allow for the immediate retrieval of the data corresponding to a single mouse model or to compare the shared and unique aspects of many polyQ models. The second data table, which is presented in another publication (Part II), covers therapeutic research in mouse models by summarizing all of the therapeutic strategies employed in the treatment of polyQ disorders, phenotypes that are used to examine the effects of the therapy, and therapeutic outcomes.

Egr-1 induces DARPP-32 expression in striatal medium spiny neurons via a conserved intragenic element

DARPP-32 (dopamine and adenosine 3', 5'-cyclic monophosphate cAMP-regulated phosphoprotein, 32 kDa) is a striatal-enriched protein that mediates signaling by dopamine and other first messengers in the medium spiny neurons. The transcriptional mechanisms that regulate striatal DARPP-32 expression remain enigmatic and are a subject of much interest in the efforts to induce a striatal phenotype in stem cells. We report the identification and characterization of a conserved region, also known as H10, in intron IV of the gene that codes for DARPP-32 (Ppp1r1b). This DNA sequence forms multiunit complexes with nuclear proteins from adult and embryonic striata of mice and rats. Purification of proteins from these complexes identified early growth response-1 (Egr-1). The interaction between Egr-1 and H10 was confirmed in vitro and in vivo by super-shift and chromatin immunoprecipitation assays, respectively. Importantly, brain-derived neurotrophic factor (BDNF), a known inducer of DARPP-32 and Egr-1 expression, enhanced Egr-1 binding to H10 in vitro. Moreover, overexpression of Egr-1 in primary striatal neurons induced the expression of DARPP-32, whereas a dominant-negative Egr-1 blocked DARPP-32 induction by BDNF. Together, this study identifies Egr-1 as a transcriptional activator of the Ppp1r1b gene and provides insight into the molecular mechanisms that regulate medium spiny neuron maturation.

Characterization of a rat model of Huntington's disease based on targeted expression of mutant huntingtin in the forebrain using adeno-associated viral vectors

Huntington's disease (HD) is a fatal neurodegenerative disorder caused by an expanded CAG repeat in the huntingtin (htt) gene. Neuropathology is most severe in the striatum and cerebral cortex. As mutant htt is ubiquitously expressed, it has not been possible to establish clear structure-to-function relationships for the clinical aspects. In the present study, we have injected recombinant adeno-associated viral vectors of serotype 5 (rAAV5) expressing an 853-amino-acid fragment of htt with either 79 (mutant) or 18 (wild-type) glutamines (Q) in the dorsal striatum of neonatal rats to achieve expression of htt in the forebrain. Rats were followed for 6 months and compared with control rats. Neuropathological assessment showed long-term expression of the green fluorescent protein (GFP) transgene (used as a marker protein) and accumulation of htt inclusions in the cerebral cortex with the rAAV5-htt-79Q vectors. We estimated that around 10% of NeuN-positive cells in the cerebral cortex and 2% of DARPP-32 neurons in the striatum were targeted with the GFP-expressing vector. Formation of intracellular htt inclusions was not associated with neuronal loss, gliosis or microglia activation and did not lead to altered motor activity or changes in body weight. However, the same mutant htt vector caused orexin loss in the hypothalamus - another area known to be affected in HD. In conclusion, our results demonstrate that widespread forebrain expression of mutant htt can be achieved using rAAV5-vectors and suggest that this technique can be further explored to study region-specific effects of mutant htt or other disease-causing genes in the brain.

Gene expression and behaviour in mouse models of HD

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease, resulting in expansion of the CAG repeat in exon 1 of the HTT gene. The resulting mutant huntingtin protein has been implicated in the disruption of a variety of cellular functions, including transcription. Mouse models of HD have been central to the development of our understanding of gene expression changes in this disease, and are now beginning to elucidate the relationship between gene expression and behaviour. Here, we review current mouse models of HD and their characterisation in terms of gene expression. In addition, we look at how this can inform behaviours observed in mouse models of disease. The relationship between gene expression and behaviour in mouse models of HD is important, as this will further our knowledge of disease progression and its underlying molecular events, highlight new treatment targets, and potentially provide new biomarkers for therapeutic trials.

Forkhead box protein p1 is a transcriptional repressor of immune signaling in the CNS: implications for transcriptional dysregulation in Huntington disease

Forkhead box protein p1 (Foxp1), a transcription factor showing highly enriched expression in the striatum, has been implicated in central nervous system (CNS) development, but its role in the mature brain is unknown. In order to ascertain functional roles for Foxp1 in the CNS, we have identified gene targets for Foxp1 both in vitro and in vivo using genome-wide expression microarrays and chromatin-immunoprecipitation followed by high-throughput sequencing (ChIP-seq) assays. We found that mouse Foxp1 overexpression in striatal cells elicited expression changes of genes related to immune signaling, transcriptional regulation and a manually curated Huntington's disease (HD)-signaling pathway. Similar results were found when the gene expression data set was integrated with Foxp1-binding data determined from ChIP-seq analysis. In vivo lentiviral-mediated overexpression of human FOXP1 in the context of mutant huntingtin (Htt) protein resulted in a robust downregulation of glial cell-associated, immune genes, including those encoding a variety of cytokines and chemokines. Furthermore, Foxp1-induced expression changes were significantly negatively correlated with those changes elicited by mutant Htt protein in several different HD mouse models, and most significantly in post-mortem caudate from human HD subjects. We finally show that Foxp1 interacts with mutant Htt protein in mouse brain and is present in nuclear Htt aggregates in the striatum of R6/1 transgenic mice. These findings implicate Foxp1 as a key repressor of immune signaling in the CNS and suggest that the loss of Foxp1-mediated gene regulation in HD contributes to the immune dysfunction in this disease. We further suggest that Foxp1-regulated pathways might be important mediators of neuronal-glial cell communication.

Huntington's disease and the striatal medium spiny neuron: cell-autonomous and non-cell-autonomous mechanisms of disease

Huntington's disease is an autosomal dominant disorder caused by a mutation in the gene encoding the protein huntingtin on chromosome 4. The mutation is an expanded CAG repeat in the first exon, encoding a polyglutamine tract. If the polyglutamine tract is > 40, penetrance is 100% and death is inevitable. Despite the widespread expression of huntingtin, HD has long been considered primarily as a disease of the striatum. It is characterized by selective vulnerability with dysfunction followed by death of the medium size spiny neuron. Considerable effort is being expended to determine whether striatal damage is cell-autonomous, non-cell-autonomous, requiring cell-cell and region to region communication, or both. We review data supporting both mechanisms. We also attempt to organize the data into common mechanisms that may arise outside the medium, spiny neuron, but ultimately have their greatest impact in the striatum.

Striatal atrophy and dendritic alterations in a knock-in mouse model of Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disease characterized by progressive atrophy of the striatum, cerebral cortex, and white matter tracks. Major pathological hallmarks of HD include neuronal loss, primarily in the striatum, and dendritic anomalies in surviving striatal neurons. Although many mouse models of HD have been generated, their success at reproducing all pathological features of the disease is not fully known. Previously, we demonstrated extensive striatal neuronal loss and striatal atrophy at 20-26 months of age in a knock-in (KI) mouse model of HD. To further investigate this model, which carries a human exon 1 with ∼119 CAG repeats inserted into the mouse gene (initially 140 repeats), we have examined whether these mice exhibit the atrophy and neuronal anomalies characteristic of HD. Stereological analyses revealed no changes in the striatal volume of male and female homozygote mice at 4 months, however striatal atrophy was already present at 12 months in both sexes. Analysis of cortical and corpus callosum volume in male homozygotes revealed a loss in corpus callosum volume by 20-26 months. At this later age, the surviving striatal neurons displayed extensive loss of spines in distal branch orders that affected both immature and mature spines. Mirroring late stage HD striatal neuronal morphology, the striatal neurons at this late age also showed reduced dendritic complexity, as revealed by Sholl analysis. Tyrosine hydroxylase immunoreactivity was also decreased in the striatum of 20-26 month old KI mice, suggesting an alteration in striatal inputs. These data further indicate that CAG140 homozygote KI mice exhibit HD-like pathological features and are a useful model to test the effects of early and/or sustained administration of novel neuroprotective treatments.

Pathophysiology of Huntington's disease: time-dependent alterations in synaptic and receptor function

Huntington's disease (HD) is a progressive, fatal neurological condition caused by an expansion of CAG (glutamine) repeats in the coding region of the Huntington gene. To date, there is no cure but great strides have been made to understand pathophysiological mechanisms. In particular, genetic animal models of HD have been instrumental in elucidating the progression of behavioral and physiological alterations, which had not been possible using classic neurotoxin models. Our groups have pioneered the use of transgenic HD mice to examine the excitotoxicity hypothesis of striatal neuronal dysfunction and degeneration, as well as alterations in excitation and inhibition in striatum and cerebral cortex. In this review, we focus on synaptic and receptor alterations of striatal medium-sized spiny (MSNs) and cortical pyramidal neurons in genetic HD mouse models. We demonstrate a complex series of alterations that are region-specific and time-dependent. In particular, many changes are bidirectional depending on the degree of disease progression, that is, early vs. late, and also on the region examined. Early synaptic dysfunction is manifested by dysregulated glutamate release in striatum followed by progressive disconnection between cortex and striatum. The differential effects of altered glutamate release on MSNs originating the direct and indirect pathways is also elucidated, with the unexpected finding that cells of the direct striatal pathway are involved early in the course of the disease. In addition, we review evidence for early N-methyl-D-aspartate receptor (NMDAR) dysfunction leading to enhanced sensitivity of extrasynaptic receptors and a critical role of GluN2B subunits. Some of the alterations in late HD could be compensatory mechanisms designed to cope with early synaptic and receptor dysfunctions. The main findings indicate that HD treatments need to be designed according to the stage of disease progression and should consider regional differences.

Huntington's Disease and Striatal Signaling

Huntington’s Disease (HD) is the most frequent neurodegenerative disease caused by an expansion of polyglutamines (CAG). The main clinical manifestations of HD are chorea, cognitive impairment, and psychiatric disorders. The transmission of HD is autosomal dominant with a complete penetrance. HD has a single genetic cause, a well-defined neuropathology, and informative pre-manifest genetic testing of the disease is available. Striatal atrophy begins as early as 15 years before disease onset and continues throughout the period of manifest illness. Therefore, patients could theoretically benefit from therapy at early stages of the disease. One important characteristic of HD is the striatal vulnerability to neurodegeneration, despite similar expression of the protein in other brain areas. Aggregation of the mutated Huntingtin (HTT), impaired axonal transport, excitotoxicity, transcriptional dysregulation as well as mitochondrial dysfunction, and energy deficits, are all part of the cellular events that underlie neuronal dysfunction and striatal death. Among these non-exclusive mechanisms, an alteration of striatal signaling is thought to orchestrate the downstream events involved in the cascade of striatal dysfunction.

Forebrain striatal-specific expression of mutant huntingtin protein in vivo induces cell-autonomous age-dependent alterations in sensitivity to excitotoxicity and mitochondrial function

HD (Huntington's disease) is characterized by dysfunction and death of striatal MSNs (medium-sized spiny neurons). Excitotoxicity, transcriptional dysregulation and mitochondrial abnormalities are among the mechanisms that are proposed to play roles in HD pathogenesis. To determine the extent of cell-autonomous effects of mhtt (mutant huntingtin) protein on vulnerability to excitotoxic insult in MSNs in vivo, we measured the number of degenerating neurons in response to intrastriatal injection of QA (quinolinic acid) in presymptomatic and symptomatic transgenic (D9-N171-98Q, also known as DE5) mice that express mhtt in MSNs but not in cortex. After QA, the number of degenerating neurons in presymptomatic DE5 mice was not significantly different from the number in WT (wild-type) controls, suggesting the early, increased vulnerability to excitotoxicity demonstrated in other HD mouse models has a largely non-cell-autonomous component. Conversely, symptomatic DE5 mice showed significantly fewer degenerating neurons relative to WT, implying the resistance to excitotoxicity observed at later ages has a primarily cell-autonomous origin. Interestingly, mitochondrial complex II respiration was enhanced in striatum of symptomatic mice, whereas it was reduced in presymptomatic mice, both relative to their age-matched controls. Consistent with the QA data, MSNs from symptomatic mice showed decreased NMDA (N-methyl-d-aspartate) currents compared with age-matched controls, suggesting that in addition to aging, cell-autonomous mechanisms mitigate susceptibility to excitotoxicity in the symptomatic stage. Also, symptomatic DE5 mice did not display some of the electrophysiological alterations present in other HD models, suggesting that blocking the expression of mhtt in cortical neurons may restore corticostriatal function in HD.

Pathogenic mechanisms in Huntington's disease

Huntington's disease (HD) is an autosomal dominant, progressive neurodegenerative disorder presenting in midlife. Multiple pathogenic mechanisms which hypothesise how the expanded CAG repeat causes manifest disease have been suggested since the mutation was first detected. These mechanisms include events that operate at both the gene and protein levels. It has been proposed that somatic instability of the CAG repeat could underlie the striatal-specific pathology observed in HD, although how this occurs and what consequences this has in the disease state remain unknown. The form in which the Htt protein exists within the cell has been extensively studied in terms of both its role in aggregate formation and its cellular processing. Protein-protein interactions, post-translational modifications and protein cleavage have all been suggested to contribute to HD pathogenesis. The potential downstream effects of the mutant Htt protein are also noted here. In particular, the adverse effect of the mutant Htt protein on cellular protein degradation, subcellular transport and transcription are explored, and its role in energy metabolism and excitotoxicity investigated. Elucidating the mechanisms at work in HD pathogenesis and determining when they occur in relation to disease is an important step in the pathway to therapeutic interventions.

The biological function of the Huntingtin protein and its relevance to Huntington's Disease pathology

Huntington's Disease is an adult-onset dominant heritable disorder characterized by progressive psychiatric disruption, cognitive deficits, and loss of motor coordination. It is caused by expansion of a polyglutamine tract within the N-terminal domain of the Huntingtin protein. The mutation confers a toxic gain-of-function phenotype, resulting in neurodegeneration that is most severe in the striatum. Increasing experimental evidence from genetic model systems such as mice, zebrafish, and Drosophila suggest that polyglutamine expansion within the Huntingtin protein also disrupts its normal biological function. Huntingtin is widely expressed during development and has a complex and dynamic distribution within cells. It is predicted to be a protein of pleiotropic function, interacting with a large number of effector proteins to mediate a host of physiological processes. In this review, we highlight the wildtype function of Huntingtin, focusing on its postdevelopmental roles in axonal trafficking, regulation of gene transcription, and cell survival. We then discuss how potential loss-of-function phenotypes resulting in polyglutamine expansion within Huntingtin may have direct relevance to the underlying pathophysiology of Huntington's Disease.